U.S. patent application Ser. No. 08/931,575, filed Sep. 16, 1997, which is hereby incorporated herein by reference, discloses the use of a printed invisible encodement on a photographic image to record sound information. The encodement is read by illuminating using a beam of invisible electromagnetic radiation that is subject to modulation by the encodement. The resulting encodement image is captured, decoded, and played back. The photographic image on which the encodement is printed is originally captured using a camera. The invisible radiation image is captured using a data reader that is capable of capturing only invisible images within a selected band. (The term "band" is used herein to refer to one or more contiguous or non-contiguous regions of the electromagnetic spectrum. The term "invisible" is used herein to describe material which is invisible or substantially invisible to the human eye when viewed under normal viewing conditions, that is, facing the viewer and under sunlight or normal room illumination such as incandescent lighting.) The invisible image is produced by development of a photographic emulsion layer, inkjet printing, thermal dye transfer printing or other printing method. The encodement is a two-dimensional array of encoded data.
Machine readable optical data codes are widely used. Optical data codes are in the form of one-dimensional codes, commonly referred to as "bar codes", and two-dimensional codes, sometimes referred to as "two-dimensional bar codes". Simple, manually scanned readers, such as that disclosed in U.S. Pat. No. 4,603,262, can be provided for one-dimensional codes. More complex readers are needed for two-dimensional codes. These readers are held over the code, while the reader internally scans the code or captures an instantaneous two-dimensional image.
Some optical code readers illuminate visible bar codes with a beam of invisible or "nearly invisible" radiation. It is well known with such readers to use a light projection to help in targeting optical codes. U.S. Pat. No. 4,603,262 and U.S. Pat. No. 4,652,750 teach the use of a projection to provide a spot during manual scanning with an invisible beam. U.S. Pat. No. 5,668,364 teaches a similar marker projection that is used, with a two-position switch, for targeting prior to scanning. U.S. Pat. No. 5,598,007; U.S. Pat. No. 5,627,360; and U.S. Pat. No. 5,550,364 teach readers which utilize a projection having one or more pair of converging beams. The readers are moved toward or away from an optical code until, at an appropriate focusing distance, each pair of beams converges into a single spot. The multiple pairs of beams can be used in aligning the readers and optical codes. U.S. Pat. No. 5,289,220 teaches the use of similar close focusing beams in a camera. Reader alignment using a two-dimensional spot pattern, such as a bar or line, or pair or spots, or scanned spot; are also taught by: U.S. Pat. No. 4,820,911; U.S. Pat. No. 5,468,949; U.S. Pat. No. 5,212,371; U.S. Pat. No. 5,486,944. U.S. Pat. No. 5,378,883 teaches an off-center spot that is brought nearer the center. The "User's Guide" for the WelchAllyn.TM. Imageteam.TM. 4400 2D hand-held image reader, marketed by Welch Allyn, Inc., of Skaneateles Falls, N.Y., teaches the use of a reader having targeting spots in the form of a cross that is centered on the optical code. Four additional spots, in the form of inwardly facing chevrons, define outer limits of the detection area. The targeting spots are produced by use of a laser diode and diffractive optical element and are extinguished during capture of a code.
In the above references, the marker projection or beams are targeted at and impinge on the subject matter of interest, a visible bar code or other code. The illumination of the code by the marker projection makes it easy to see the code in dim conditions, but makes it difficult to clearly view the details and coloration of the code, since the illumination is uneven and generally monochromatic. This is inconsequential, however, since the code only needs to be targeted by the user, not viewed.
Laser pointers are available which use a laser diode and diffractive optical element to project a wide variety of patterns. Diffracted patterns sometimes present a rectangular outline or other array of shaped features surrounding a residual undiffracted spot from the zero order beam.
It would be desirable to provide a data reader and system for reading an invisible image under lighting conditions suitable for viewing an underlying visible image, in which a diffracted, centerless aiming projection is free from a zero order beam or beams.